1. Field of the Invention
The present invention relates to a pseudo-noise generating apparatus for generating a pseudo-noise sequence used in the spreading modulation of a transmission signal in a CDMA (Code Division Multiple Access) communication system.
2. Description of the Related Art
In a CDMA communication system which uses a spreading code sequence whose period may extend to, for example, as much as several minutes, code synchronization must be established quickly by starting the spreading code sequence from the phase determined based on time data that the transmitting end sends through a control channel. Further, in a system in which a code sequence of a long period is divided into a plurality of sections each for use by a different station, it is required that the code sequence be quickly started from the phase to be used, and that when an assignment change request is issued to reduce mutual interference, the code sequence be restarted quickly from the assigned phase.
One possible method to arbitrarily set the phase of the pseudo-noise sequence used as a spreading code sequence is to supply a faster-than-usual clock to the pseudo-noise generator that generates the pseudo-noise sequence and shift its phase from the initial phase to the desired phase. This method, however, requires a high-speed clock to accomplish the shifting, and presents a problem particularly when the shift amount is large, because a very fast clock becomes necessary if the desired shift amount is to be obtained within a short time.
Another possible method is to utilize the shift additive nature of the pseudo-noise sequence and generate a phase-shifted code sequence by taking appropriate taps from the respective stages of a shift register in the pseudo-noise generator and EORing the tapped outputs. This method, however, has the problem that the tap positions for obtaining the desired shift amount must be determined by calculation and, if the number of shift register stages is large, it takes considerable time for calculation.